Referring to FIG. 1, a Numerical Control machine, or NC machine, is a cutting machine with a numerical control unit 150 that guides a path of a cutting tool (e.g., a cutting torch 125), above a bed 140 on which a generally flat sheet or plate 160 rests. Generally, NC machines cut a series of shaped parts from the sheet/plate 160 with a vertical torch.
Cutting with an NC machine is controlled by an NC program which resides in NC control unit 150. This NC program is typically a user-readable list of coordinates and commands using the letters A-Z and the numbers 0-10. Additional characters usually include a period (“.”), positive (“+”), and/or negative (“−”) signs. Orthogonal planar axes of the NC machine are usually designated by the letters “X” and “Y,” although in some NC languages the designation is implied by position in the control line of text, the first being “X” and the second number being “Y.” Such a list of movements and actions is known as an NC program. NC machines are used extensively in manufacturing industries, and typically follow a version of Electronic Industry Standard 273 known as Word Address, or the equivalent European Standard known as ESSI, as defined by International Standards Organization as ISO 6582, and which uses only numbers 0-9, signs “+” and “−,” and implicitly reads the X and Y axes through a position in an expression such as +100+200, where the X movement is 10 mm and the Y movement is 20 mm.
Some NC machines, such as in embodiments discussed below, are known as “profiling” machines, which cut shapes from a flat sheet or plate material with torches 125, which can include oxy-acetylene, plasma, laser, and/or water jet.
Torch 125 is generally moved vertically using a motorized Torch Height Control (THC) 130 mounted on a gantry 135. The THC 130 allows the NC machine to cut materials of various thicknesses by retracting and lowering torch 125 to the material 160 to be cut. Generally, while cutting shapes in two dimensions, it is important to control the height of torch 125 above material 160.
Machines such as that shown in FIG. 1 are typically engineered to be flat and horizontal. Bed 140 on which the plate/workpiece lies is also expected to be flat, although bed 140 tends to become damaged over time with use. The plate is considered substantially close to “flat” originally, and can be further flattened before cutting if required. The rails on which gantry 135 runs are also supposed to be substantially flat and horizontal. Thus, all surfaces are expected, in principle, to be near flat, horizontal, and parallel from the start of operations. In theory, it would thus be conceivable that a clean new machine with a flat bed should be capable of cutting a plate without using height control feedback. In reality, height control feedback can be avoided only for short distance cuts. The risk of the cutting head colliding with the plate is much too high in practice for large parts and long distance cuts.
When cutting parts with beveled edges, on the other hand, maintenance of the height of the head becomes more critical than when edges of a cut part are not beveled. A torch height 210 is illustrated in FIG. 2, for example. Laser torches are commonly used to cut thinner materials, in the thickness range 0-10 mm, for example, although some laser torches are used to cut steel up to 50 mm thick. Torch height 210, or the torch-to-plate cutting distance, is typically not more than a few millimeters when using a laser torch. Plasma torches, on the other hand, are commonly used to cut thicker materials, in the thickness range 0-50 mm, and sometimes as thick as 160 mm. The torch-to-plate distance with a plasma torch is typically 4-10 mm. Oxy-acetylene torches and water jets are commonly used to cut materials as thick as 400 mm, with torch-to-plate distances in the order of approximately 20 mm.
For systems where a vertical torch is used to cut a sheet or plate, it is important to set and maintain torch height 210 at an optimum point above the material, which can be done by a Torch Height Control (THC) system 220, together with a motor 200 (best seen in FIG. 2), which raises and drops a torch 205 as required.
Another use for THC system 220 is to avoid collisions with the plate material, or pieces cut from the material that may have moved or broken from it. At times, even the scrap plate skeleton itself can spring or bend and hit the torch 205 as it moves across the material. Height control thus can be used both to prevent damage to the torch, and to optimize the height of the torch, and therefore the quality of the cut.
The torch height 210 is maintained by a separate feedback loop independently of NC control (CNC) 260 and programmable logic controller (PLC) 270, which generates X axis 280 and Y axis 290. THC system 220 works on measuring torch height 210 during cutting of the plate or material. When plasma cutting, for example (as shown in FIG. 2), torch height 210—while cutting—is maintained by measuring a voltage of a plasma arc that is directly related to its arc length. The voltage is supplied by a power supply 230 to THC system 220, which in turn drives motor 200 to adjust the height of torch 205 to keep both the voltage and torch height 210 within desired bounds. This feedback system is used almost universally for plasma torches, and is independent of NC control (CNC) 260 during operation. Such technology is commonly known as Automatic Voltage Control (AVC), and removes a need for an independent height sensing device. AVC systems are convenient for plasma arc cutting because interference between sensitive electronics in a sensor used in a different measurement system, in the presence of a plasma arc, might be rendered ineffective. Other continuous height sensors include lasers, acoustic devices, capacitive ring sensors, and/or various plate riders that drag or roll over the plate surface.
The separate and independent height control loop between a sensor and the THC 220 allows NC control 260 to concentrate processing power on other functions that are of higher priority, which can be significant when processing power is limited. Movement of torch 205 to maintain torch height 210 is in principle a consequence only of a bent plate, or that the plate and machine are not both horizontal and perfectly parallel to each other. For this reason, torch height 210 is to be independent of the conventional NC program, which is only concerned with XY movement of a vertical torch 205, and not at all with movement in the vertical or Z direction. Thus, communication between NC control 260 and THC 220 is usually restricted to general functions such as “Raise,” “Lower,” “Height Control On,” and “Height Control Off.” In most known profiling machines, NC control 260 does not directly control the exact height, Z, during cutting.
Conventional height control systems tend to work adequately for simple vertical cutting (i.e., where a cutting torch is aligned perpendicular to the workpiece), though the height of the torch relative to the workpiece may vary within a considerable distance range. For plasma cutting, the variance may typically be +/−3 mm. For vertical torch cutting, the variance tends to only affects the quality of the cut, but will have little effect on the shape of the part that is cut.
Mechanical devices that physically contact the plate for cutting are generally more accurate than AVC devices, but mechanical devices are known to experience problems when their cutting heads run over previously cut paths, or fall into holes in the workpiece. Such difficulties can require very complex NC programs to avoid these obstacles to efficient cutting. Ball bearing mechanical feet, for example, can become clogged with dirt or metal from the surface of or waste from the workpiece.
The conventional relationship between an arc voltage (e.g., sensed arc voltage 240) for plasma cutting and the respective torch height is known to be only a general range, and not a specific value, because the AVC arc is unstable at the start of cutting, and even when the arc “stabilizes,” the required voltage for the AVC must still be averaged because of continual variations in the arc for reasons other than variation of torch height 210. AVC is also known to be less useful for cutting small holes and tight corners, because the typical fluctuations in arc conditions can produce unfavorable torch height changes, thereby resulting in lower quality cuts.
One significant drawback to using conventional feedback THC systems, such as that shown in FIG. 2, is that when torch 205 traverses a hole in the material, torch 205 can “dive” (i.e., drop into the hole). The dive results from the THC system are unable to distinguish between the hole versus a simple variation in the height of the material of the workpiece. Conventional systems deal with such problems by turning off the height control in the NC program at various points in the programmed cutting path, for example, such as when torch 205 moves near previously cut holes or paths, or when cutting into scrap material of the workpiece which may have tilted or dropped. Torch dives can result in poor quality cutting, and also in damaging collisions between the torch and the workpiece material.
Torch dives are difficult to avoid because the cutting path of a part will invariably encounter a hole at least near the exit from the cutting path to drop the cut part from the workpiece material. Similarly, a void in the workpiece can also be encountered in the cutting path when a piece of adjacent scrap material may have already fallen out of the workpiece from an earlier cut. Accordingly, many NC programs further rely on generating complex machine paths to avoid traversing previously cut holes, or else the programs force full torch lifts after every part or hole is cut. Because torch lifts can slow the cutting process significantly, full torch lifts for each part or hole can add greatly to the time needed to cut and process a nest of parts, or to traverse a pattern of holes in one part. As the variations in plate shape are not known before cutting, such conventional slowing of processes is performed even when the workpiece material is, for all intentions, reasonably flat, and when THC variations are largely irrelevant to vertical cuts.
Turning now to FIG. 3, when cutting is not only vertical, some more complex machines allow a torch 305 (e.g., torches 125, 205 of FIGS. 1, 2, respectively) to be tilted under program control to create a beveled edge to a cut part. Multiple cutting passes on a workpiece, each at a different tilt angle to the torch, as shown in FIG. 4, can create multiple bevels to an edge of the part. Beveling is of great commercial interest because most cut materials have to be subsequently welded to other parts, which in the art is typically a slow, difficult, messy, and expensive manual process. The welding process though, can be simplified and made more efficient by first performing the beveling operations on the part during the cutting process, as best illustrated in FIG. 4.
In edge beveling operations, however, the greatest problems are known to occur with independent THC systems that are based on continual feedback from height sensors while cutting. Referring back to FIG. 3, the present inventor has discovered that conventional height control systems move the torch only in a vertical direction 300, in order to maintain an arc length 310, rather than along the axis 340 of torch 305. Therefore, any movement in the vertical direction 300 without concurrent associated plate/workpiece movement will change a desired intersection point 320 for cutting to a different, and incorrect, point 330 on the plate surface (i.e., at intersection point 320). When such a shift occurs, the angle of torch 305 will still be correct, but the position of the cut on the plate/workpiece will be incorrect. As an example, at 45 degrees of inclination to torch 305, a vertical displacement of 1 mm in direction 300 (to maintain arc length 310) will result in a horizontal displacement of 1 mm on the workpiece between points 320 and 330.
Accordingly, conventional beveling machines experience unplanned vertical movement of the torch, i.e., other than for a variation of torch-to-plate distance 210 (see FIG. 2), resulting in incorrect cuts of the outline of respective parts. For example, in AVC systems, the arc length/voltage 310, which does roughly vary with torch height, is not an accurate absolute measure of torch height. Also, the arc length/voltage 310 is affected by many other factors, including the absence of previously removed material, the proximity to previous cuts, or other factors related to normal variation arc conditions that are unrelated to torch height. These factors are not considered to cause problems for vertical cutting applications, but will certainly lead to significant accuracy problems in bevel cutting, as point 330 is not the desired point 320.
Referring again to FIG. 4, bevel cutting for weld preparation can involve one, two, three, or more passes of the torch. An example of three-pass beveling is shown represented by an under cut 400, a center cut 410, and a top cut 420, which all may be performed in this respective order. A “K”-shaped bevel preparation is shown, and can be used for subsequent welding of the cut part from plate. For the cut part to be usable, the tolerance of dimensions 430, 440, and 450 should be within +/−1 mm. If, however, the THC varies within the typical +/−3 mm, as discussed above, such tolerances are impossible to achieve.
Because of such difficulties in achieving desired dimension tolerances for beveled parts, the use of NC flame, waterjet, and plasma machines for preparation of weld-ready parts has been historically unacceptable. These types of machines thus represent less than 1% of all machines sold. As the present inventor has discovered, plasma machines being used for multi-pass weld preparation are rare, and even more rarely work satisfactorily, even for a single-pass beveling operation. Attempts with such machines to do any cutting other than vertical are often abandoned. The largest manufacturers of such machines have attempted to produce multi-pass bevel machines for many years, but most have abandoned AVC based systems as unworkable.
As discussed above, for theoretically flat machine beds and plates, short cuts can be reasonably performed without requiring height control feedback. However, in practice, limiting the cuts to only short distances and/or small workpieces is not practical or economically feasible.
An alternative approach that has been attempted is the measurement of the profile of the plate after it is loaded, but before it is cut. The Messer Greisham company, for example, is known to have implemented a concept of prior measurement of particular XYZ points on a workpiece before cutting the workpiece. In the Messer Greisham approach, commands are inserted in the NC program to move the cutting head of the torch to predetermined points along an exact path to be cut, but prior to the actual cutting itself. The voltage is measured at each of particular XY points on the workpiece, and stored in the memory of the NC control 150 for each measured XY point. When the machine again reaches these same XY points during cutting, the previously measured AVC voltage is recalled in the NC program to approximate a height value from the recalled voltage value. These XY points are chosen by the NC programmer during creation of the NC program.
According to the Messer Greisham approach, the XY points must be chosen by the creator of the NC program prior to cutting, and then embedded in the NC program. These points must then be explicitly recalled during cutting, and at the same respective locations. The approach thus requires skilled judgment by the programmer on general principles, without prior knowledge of the shape of the plate workpiece. In a multi-pass situation, where the paths are near parallel but slightly different, the required skill of the programmer increases substantially. A typical programmer is not generally considered capable of programming the NC machine to perform within required tolerances. The coding of the NC program must also be completed before nesting occurs, which factor is a significant impediment to the nesting process itself. Substantial changes must be made to the NC control to accommodate geometric transformations of individual bevels as part of the program. The approach uses samples of the arc voltage measured at specific points, which, as noted above, is not a direct measure of torch height.
While the Messer Greisham approach can be better than the dynamic feedback approach in beveling operations, because the Z height is theoretically “correct” at the start of each machine movement, a problem still occurs where the approach of the torch is based on the assumption that Z varies linearly between the points that are actually measured in advance of the cutting operation. As discussed above, many factors can drastically reduce the feasibility of such an assumption. When a workpiece plate is substantially flat within desired tolerances, and the flatness is not significantly altered during the cutting process, the Messer Greisham assumption can be sufficient. However, the present inventor has discovered that, in practice, when long or curved cuts are made, the flatness and parallelism of the respective plate, bed, and machine will vary on the order of 10 mm, and over a run of only a few meters. Accordingly, the Messer Greisham approach can create dangerous risks to the machine parts, since distance between the workpiece and the cutting head, for example, is not a straight line over the entire cutting, thereby resulting in a significant risk of a collision between plate and torch.
The Messer Greisham approach further requires the use of special codes in the NC program, which makes the programming nonstandard, thereby requiring additional extensions to most standard NC languages, especially to the ESSI code discussed above, as well as additional costs and resources to implement the approach effectively. The Messer Greisham approach also does not appear to use a directly programmable Z axis (see element 745 in FIG. 7, discussed further below), but instead simply utilizes the arc voltage that was previously recorded from the NC program (see element 260, FIG. 2) to a traditional THC (see element 220, FIG. 2). The Messer Greisham approach uses no element of prediction; it instead measures and remembers specific voltages around the path to be cut.